1. Field of the Invention
The present invention relates to a rotation detection apparatus, which is also referred to as “a rotary encoder”, and particularly to a rotation detection apparatus used for control of a motor.
2. Description of the Related Art
Accurate control of a rotational amount or speed of a motor is made by a feedback control that feeds back an actual rotational amount or speed detected by a rotary encoder and controls the motor so that the detected rotational amount or speed becomes closer to a target rotational amount or speed. The rotary encoder includes a rotatable member that rotates with rotation of the motor, and a sensor that magnetically or optically detects a pattern for rotation detection (rotation detection pattern) provided to the rotatable member.
The rotation detection pattern is formed by multiple pattern element portions which are S- and N-pole magnetized portions or light-transmissive/reflective and non-light-transmissive/reflective portions) arranged in a rotational direction of the rotatable member. Arranging these pattern element portions accurately at designed positions on the rotatable member enables an accurate detection of the rotational amount or speed of the motor. However, a manufacturing error in the rotatable member often shifts the arrangement of the pattern element portions from their designed positions, which is called an arrangement error.
Japanese Patent Laid-open No. 10-229691 discloses, in order to remove influence of an error in a magnetization pitch in a driving magnet of a motor, a method acquiring a period from an edge interval between a change edge of an encoder output to another change edge temporally precedent thereto by a predetermined number of edges and calculating a rotational speed on a basis of the period.
However, the method disclosed in Japanese Patent Laid-open No. 10-229691 calculates the rotational speed without using neighboring pattern element portions, which degrades a detection resolution of the rotational speed and thereby disables detection of a minute oscillation of the rotational speed.